Every sandlot player who has thrown out a runner or chased down a fly ball has a working knowledge of how a baseball travels through the air. But why does the ball move in the ways it does and how is it affected by environmental conditions?

Let's start with the ball itself. A baseball is constructed of a small cork center coated with rubber. This is tightly wrapped with many layers of yarn. Finally, a figure-eight shaped leather cover is sewn on. The stitches, or seams, protrude slightly above the surface of the ball. The finished ball is 9 inches in circumference, weighs five ounces, and has 108 stitches.

A baseball rarely travels in a straight line. This is due primarily to the seams; the ball is neither perfectly spherical nor smooth. To illustrate this, consider the curveball. If the pitcher spins the ball as he releases it, the seams on one side of the ball move toward the direction of travel, while those on the other side are moving away from it. For example, a right-handed pitcher typically spins the outside or right side of the ball toward the batter. As the ball rotates in flight, the seams passing along this right side push into the wind. Conversely, as the seams rotate past the direction of travel and along the left side, they are moving with the wind.

This creates a higher air pressure, or stronger wind, along the right side of the ball and a region of lower pressure along the left side. This pressure differential pushes the ball to the left and the pitch curves in that direction. Similarly, a left-handed curve ball moves to the right. In the same way, rotating the bottom of the ball toward the hitter results in a rising pitch and rotating the top toward the hitter will cause the ball to drop.

The faster the pitch, the less time the pressure differences have to act on the ball resulting in a curve that begins further along the path to the hitter. In this family of pitches called sliders, the ball doesn't curve much until it approaches the batter, where it suddenly curves and drops, leading him to believe he is facing a fastball until that last-moment swoop away from his bat.

So one might think the fastball is a pitch that moves in a straight line. But even a fastball has some rotation or spin, though not as much as a curve ball or slider. A well-thrown fastball typically has quite a bit of non-linear movement. But unlike the vexing arc of the curveball or the trickery of the slider, the primary purpose of the fastball is to get past the hitter as quickly as possible.

A variety of grips, arm movements, and other techniques are combined to influence the speed of the pitch and the speed and orientation of the spin. This produces a tremendous range of ball movements, leading to a variety of pitches such as sinkers, cutters, forkballs, two-seam fastballs, change-ups, etc. The difficulty of mastering a repertoire of pitches and consistently throwing them accurately may account for the extravagant salaries of major league pitchers.

There is one other pitch worth mentioning: the knuckleball. Despite its name, the knuckleball is usually gripped with the fingertips, and thrown with a pushing motion in an attempt to produce a pitch with no spin at all. This results in very complex air turbulence effects over the seams of the ball including vortices, or tiny tornado-like turbulence patterns. The knuckleball may go in any direction, more than one direction, or even follow a corkscrew path on its flight toward the batter. The pitcher himself is unlikely to know how the ball will move, and it is by far the most difficult pitch to catch.

A batted ball is of course subject to the same laws of physics as one thrown. As the bat contacts the ball it compresses and deforms it, but the ball quickly springs back to its original shape. An object that behaves in this way is called "elastic" and it is in part this elasticity that causes the ball to leave the bat at high speed.

In addition to its spin causing the batted ball to curve, the longer flight of a fly ball allows the forces of wind and gravity to play more significant roles. In a vacuum, the ball would travel along a perfect arc, or parabola, symmetrical on either side of its apex. But due to the resistance of the air through which it passes, the ball travels further horizontally away from the hitter during the initial, upward portion of its flight than it does during the downward journey.

Wind is typically a fairly steady force against the ball, lengthening or shortening its flight and/or pushing it left or right. Of course the harder the ball is hit and the flatter its trajectory, the less time wind has to act on it and the influence of the wind is reduced accordingly.

A baseball travels farther on a warm day than a cold one, other factors being equal, because warm air is less dense than cold air and offers less resistance to the passage of the ball. In addition, higher temperatures make the ball itself slightly more elastic. Balls also travel farther at high elevation than at sea level. Again, this is due to the reduced density of the air.

Hitters look forward to playing in mile-high Coors Park in Denver for this reason. They enjoy another advantage over pitchers there as well: curve balls curve less at higher elevation, because less air resistance results in a smaller pressure difference from one side of a spinning ball to the other.

These effects can be significant. A ball traveling 350 feet on a chilly day in San Francisco may travel 380 feet or more if struck with the same force on a hot day in Denver. Humid air is also less dense than dry air, but high humidity reduces the elasticity of the ball, so the effect is negligible.

Finally, let's look at some ways players cheat to gain an advantage. Pitchers sometimes use saliva, Vaseline, or some other foreign substance on the ball. The so-called "spitball" was outlawed in the 1920s, but is probably still thrown by a few pitchers. The intent of the spitball pitcher is to make the ball heavier on one side than the other. This alters its aerodynamic properties, and causes it to curve in unexpected ways.

Similarly, some pitchers may covertly abrade or "rough up" the ball or cut a stitch. These damaged areas provide additional air resistance, magnifying the movement of the pitch. Pitchers have been caught with sandpaper, emery boards, nail files, and a variety of similar materials and instruments.

Hitters have also cheated in attempts to gain an advantage. The corked bat is perhaps the most common example. A hole is drilled into the end of the bat and filled with cork or rubber, then disguised by gluing in a wood plug. Several players have been caught when their corked bats broke on impact with the ball.

However, there is no convincing evidence that a corked bat delivers greater hitting distance. While it may increase bat speed slightly (giving the hitter a split second longer to commit to a swing), its reduced mass may offset this and actually impart less, not more, force to the ball. There is still considerable disagreement on this issue, but it is possible these players are risking ejection, suspension, and fines in pursuit of a myth.